1. Technical Field
This invention relates generally to heating chambers and in particular to a heating chamber for use in diffusion reactors for semiconductor production.
2. Description of the Prior Art
Heating chambers have been in use throughout the history of the semiconductor industry for processing silicon wafers. Heat is generated by means of electrical current passing through resistance wire within the heating chamber. With time and improvement in technology, silicon wafer size has grown to eight inches in diameter, and may grow yet larger in the future. Processing of such larger-sized silicon wafers requires a heating chamber with increased inner and outer diameters. This increase in size amplifies the problems associated with resistance wire and construction of heating chambers. Therefore, better engineered heating chambers with improved reliability and longevity are in demand.
Existing heating chambers are typically made of helically wound or sinuated resistance wire placed in the inner circumference of an insulated heating chamber. Ceramic separators are used to keep the wire from coming in contact with the adjacent turn.
The most common problem associated with heating chambers of this construction is the growth of the resistance wire with usage and time. As this wire heating element cycles between higher and lower temperatures, its linear length increases. With time, this cycling causes continuous wire growth. Prior art designs do not provide any space for growth of the wire heating element. Thus, as linear length increases, the wire is forced to buckle and deform towards the center of the heating chamber. This phenomenon, known as "wire sagging" eventually prevents the insertion of the quartz process tube, and necessitates the replacement of the heating chamber.
FIG. 1 is a partial cross-sectional view of the prior art heating chamber before usage. The heating chamber 1 has a stainless steel outer shell 2 with an inner lining of insulation 4. The wire heating element 6 is held against the insulation lining by ceramic separators 8.
FIG. 2 is a partial cross-sectional view of the prior art heating chamber after usage. Repeated temperature cycling causes the wire heating element to elongate and eventually deform 10 towards the center of the chamber.
Homer et al, Vertically Positioned Transfer System for Controlling and Initiating the Flow of Metered Amounts of Solid Materials, U.S. Pat. No. 4,668,478 (May 26, 1987) discloses a hopper with heaters connected to its outer walls. These heaters de-energize when the internal temperature reaches approximately 180.degree.. By contrast, the heating chambers used in silicon wafer processing are designed to operate at temperatures between 100.degree. and 1600.degree. C. Additionally, the heating chambers used in the silicon wafer manufacturing process have internal, rather than external, wire heating elements.
It would therefore be a significant advance in the art to provide a heating chamber and separator designed to accommodate the linear growth of the wire heating elements, and thus extend the useful life of the heating chamber.